The present invention relates to a method and apparatus for forming a cast-in-place support column. In particular, the present invention is a method and apparatus for forming a cast-in-place concrete support column below the surface of the ground.
Foundations piles or support columns are typically utilized in supporting structures such as bridges, piers, homes, industrial plants, or other similar structures. Such support column or piles are preferably formed of concrete and utilize a minimum of steel reinforcing members. A bore is formed in the ground of the desired depth and at the preferred location of the concrete support column. The concrete fill material is then introduced into the bore thus formed and allowed to cure or harden.
A number of prior art methods and apparatus have been proposed for forming a cast-in-place support column. In one such prior art system, an auger device is utilized to form the bore followed by the introduction of concrete into the bore. This method is particularly susceptible to foreign matter such as dirt separating from the walls of the bore and intruding into the column of liquefied fill material before the fill material cures. Such intrusions are particularly detrimental to the support capacity of the pile. When such intrusions do occur, the support column is, in effect, useless and must be replaced. Considerable amounts of time and expense is expended in initially forming the support column and then removing the column into which intrusions are detected, and then replacing the support column. Typically, the defective column can only be removed by massive excavation at the support column site.
In another prior art method and apparatus, a mandrel is provided and is driven into the ground to form the support column bore. During the driving of the mandrel, the concrete fill material in liquefied form is allowed to flow by gravity into the bore being formed. It has been found that the pressure of the gravity feed of fill material may not effectively prevent earth from breaking away from the wall of the bore and intruding into the liquefied fill material as the bore is being formed. This prior art method is thus also plagued by the potential problems of intrusions into the support column. Additionally, the mandrel must be extracted utilizing a pile driving hammer rigged for extraction.
In a third prior art method and apparatus, a hollow tube or mandrel is driven into the ground to form the bore for the column. The mandrel is then filled with liquefied concrete material and the tube is then extracted from the bore by the upward and downward blows of a pile driving hammer. The mandrel has a tamping effect on the concrete material as it is extracted. In this method, the liquefied concrete is compressed by the tamping action which occurs typically at the rate of 80 times per minute. This method, however, has the disadvantage of requiring relatively expensive equipment to achieve the tamping effect which also results in a high energy expenditure and therefore expense in forming the cast-in-place pile.
These disadvantages of the prior art methods and apparatus are overcome by the present invention which provides a mandrel for forming a bore and introducing liquefied fill material into the bore under pressure to extract the mandrel from the bore. The pressure of the fill material acts against the wall of the bore to prevent the formation of intrustions in the support column as the mandrel is being extracted and fill material is being introduced into the bore. Since the pressure of the concrete fill material itself acts against the mandrel to force the mandrel upward out of the bore, in the vast majority of cases and soil conditions the present invention eliminates the relatively expensive procedure of extracting the mandrel with an appropriately rigged pile driving hammer.